As far as I know, the "devanagari" package does not have support for Roman Transliteration. You should try the outdated, yet capable sanskrit package by Charles Wikner which has full support for Devanagari and Roman Transliteration.

NB - The skt pre-processor comes ONLY in source form and you'll have to compile it for your operating system. If you use Windows, you can either use VisualStudio (if you have it) or dowload a lightweight compiler like DJGPP which is only a few megabytes. Compiling the executable from the source file is outiside the purview of this question. If you have a problem with compiling the source, please post another question.